The present invention relates to a radiation-polymerizable mixture comprising as essential constituents (a) a compound having at least two terminal ethylenically unsaturated groups which can form a crosslinked polymer by means of free-radical initiated chain addition polymerization (b) a polymeric binder and (c) a radiation-activatable polymerization initiator. The radiation-polymerizable mixture of the invention is particularly useful as a dry film for the production of solder masks.
Single-faced, double-faced and especially through-hole plated printed circuit boards are provided with a solder mask before they are soldered. The application of such masks leaves only soldering pads exposed, whereby tin is conserved during soldering. In addition, during the soldering process some of the soldering heat is kept away from the workpiece so that heat-sensitive components, with which the printed circuit boards are equipped, are not damaged during the soldering. Last but not least, solder masks have the important function of covering all those areas of the circuit diagram which must not come into contact with the solder in order to avoid the possibility of forming conducting bridges between individual conducting paths, which are fatal for a circuit system.
To satisfy these quite complex requirements, various processes and various forms of products have been used in the past.
From the foregoing, it is clear that solder masks must have a pattern which is oriented according to the given conducting paths. One widely used technique is to apply solder masks by screen printing. It is also possible by means of screen printing to apply masks which have a relatively high layer thickness, thereby ensuring that the conducting paths are thoroughly embedded and well shielded.
Masks produced by screen printing processes are inherently limited by the nature of the screen printing process. When the distances between conducting paths are reduced, the relatively coarse image structures of screen printing are no longer adequate to protect the extremely fine circuit patterns of the latest microconductor technology.
In recent years solder masks have therefore been produced to an increasing extent by photoresist techniques. As in the production of printed circuits, thermoplastic photopolymerizable layers are laminated by pressure and heat to the printed circuit board and cured by imagewise exposure at those places where the printed circuit board is to be covered. The solder mask is then obtained by washing away the unexposed parts of the thermoplastic layer. This process makes possible higher image resolutions than the screen printing process.
Materials of this type are described in German Offenlegungsschrift No. 2,747,947. They comprise photopolymerizable layers which contain a certain amount of bonded halogen to improve their flame-resistance. Published European Application No. 15,004 describes a similar material which can be developed dry by mechanically separating exposed and unexposed layer areas ("peel-apart process"). Finally, published European Application No. 2,040 describes a light-curable material for the same purpose in which the light-sensitive compounds are identified as light-sensitive epoxy resins but are not described in more detail.
Photopolymerizable mixtures known and used for producing photoresist masks are transferred dry and with heating onto the printed circuit board to be masked. They must, therefore, necessarily be thermoplastic. However, for use as solder masks this thermoplasticity is disadvantageous since the masks must withstand temperatures above 200.degree. C. without decomposition and without melting or even softening to too high a degree. Admittedly, the photopolymer layer is hardened to a considerable extent in the exposed areas by crosslinking polymerization. The initial light-hardening can be further enhanced in a known way by re-exposure of the developed image stencil. However, the basic tendency of all known photopolymerizable layers to soften when heated remains.